1. Field of the Invention
The invention relates to wafer temperature control apparatus for semiconductor wafer processing systems and, more specifically, the invention relates to a heat exchanger apparatus for a semiconductor wafer support in a semiconductor processing system that maintains the wafer at a substantially constant temperature during processing of the wafer.
2. Description of the Related Art
Semiconductor wafer temperature control apparatus is useful in a variety of applications, particularly in the manufacturing of semiconductor devices using processes that require the wafers to be maintained at stable temperatures. In particular, a resistive heater may be utilized in, for example, chemical vapor deposition (CVD) processing chambers to heat a wafer during processing and maintain the wafer at an elevated temperature to facilitate film deposition.
Typically, a heater assembly is a portion of a semiconductor wafer support (known as a susceptor) and is comprised of a platen that is fabricated of aluminum or other thermally conductive material with a top surface having a generally circular shape for supporting a semiconductor wafer within a process chamber. The wafer support also includes a shaft which is coupled to the bottom of the platen and supports the platen in the process chamber. A heating element is mounted in or under the platen and is arranged to be in thermally conductive contact with the surface of the platen such that a wafer supported by the platen can be heated during processing.
One manufacturing technique for a heater assembly is a cast-in method wherein the heating element is cast-into the platen as the platen is formed, i.e., a resistive heating element is inserted into molten platen material (aluminum) during the platen manufacturing process. To accomplish the insertion of the heating element into the molten platen material without melting the heating element, the melting point of the heating element must be substantially higher than the melting point of the platen material. This means that if stainless steel or aluminum are used as the heating element, the types of materials which may be used for the platen are limited.
Another manufacturing technique for a heater assembly is a machined platen manufacturing technique wherein the heating element is placed in a channel that is milled into the bottom surface of the platen. The channel is filled with a high temperature epoxy and a cover plate is then secured (welded) over the channel to seal the heating element and epoxy into the platen. Unfortunately, the high temperature epoxy has a limited life expectancy over which it sufficiently supports the heating element and provides good thermal conductivity to the platen. Over time, the epoxy breaks down and ceases to be a good thermal conductor. As such, the useful life of a platen is limited by the useful life of the heater element and its mounting structure.
Therefore, there is a need in the art for an improved temperature control apparatus that provides effective thermal conductivity between a platen and a heat exchange element (e.g., a heating element) without the use of epoxy or without strict limits on the materials used to fabricate the platen and heat exchange element.
The invention comprises a heat exchanger apparatus including a heat exchange element coupled to a substrate support. To retain the heat exchange element within the substrate support, a clamp member is coupled to both the heat exchange element and the substrate support. To assemble the heat exchanger apparatus, a thermal differential is created between the clamp member and the substrate support such that the clamp member is enlarged to circumscribe the substrate support. Illustratively, the clamp member is heated to expand it to an enlarged state sufficient to surround the substrate support and the heat exchange element, and then the clamp member is cooled to couple the clamp member to the substrate support.
In an alternative embodiment, the substrate support comprises a platen, being substantially cylindrical, with a substantially flat wafer support surface and a bottom surface. The bottom surface contains a channel into which a heat exchange element is inserted and clamped, as discussed above, using a clamp member. In this embodiment, a corkscrew channel is formed about the outer surface of the cylindrical platen. A seal member, being a substantially cylindrical ring, is heated to expand the ring, the ring is positioned about the corkscrew channel and then the ring is cooled to affix the seal member about the corkscrew channel. The seal member in combination with the corkscrew channel forms a corkscrew conduit. The clamp member is fitted over the seal member to retain the heat exchange element in the platen as discussed above. The corkscrew conduit is useful for providing an inert gas, having a temperature defined by the temperature of the platen, to the backside of the wafer located on the wafer support surface of the platen.